The invention is directed to a receiver for the reception of angle-modulated/angle-keyed carrier signals.
Receivers of the type referred to above are utilized everywhere in communications technology where an HF signal serving as carrier and connected by modulation with an analog or digital LF signal containing the information to be transmitted is in turn to be edited. A distinction is made between an analog or digital modulation or, respectively, demodulation type dependent on the employment of an analog or of a digital LF signal. The term "keying" is employed for the digital modulation or, respectively, demodulation for distinguishing between the two types.
There are respectively different modulation or, respectively, demodulation forms for each modulation or, respectively, demodulation type (analog or digital). A distinction is thereby made between an amplitude, frequency and phase modulation or, respectively, amplitude, frequency and phase demodulation. Over and above this, thereby are numerous derivatives of the aforementioned modulation or, respectively, demodulation forms (for example, GFSK, GMSK, etc.), particularly given the digital modulation or, respectively, demodulation type. Frequency and phase modulation or, respectively, frequency and phase demodulation is also referred to as angle modulation or, respectively, demodulation.
The above comments refer to a single HF signal to be modulated or, respectively, demodulated that is available to a limited subscriber circle for the message transmission in a message system, for example in a mobile radiotelephone system or cordless telecommunication system.
In order to augment the subscriber circle, the plurality of dimensions for the analog or digital modulation or, respectively, demodulation is increased. The time and/or frequency domain are preferably utilized therefor. Alternatively thereto, it is also possible to additionally utilize the transmission channel defined by the time and frequency domain with different codings. In the utilization of the time and/or frequency domain, one speaks of a TDMA and/or FDMA method (Time Division Multiple Access; Frequency Division Multiple Access). In the utilization of the time and frequency domain in conjunction with the employment of different codings, one speaks of a CDMA method (Code Division Multiple Access).
Receiver architectures for receiving angle-keyed carrier signals whose frequencies lie in a frequency band between 890 MHz and 960 MHz given a GSM system and in a frequency band between 1880 MHz and 1900 MHz in a DECT system are therefore utilized in mobile radiotelephone technology according to the GSM standard (Groupe Speciale Mobile or Global System for Mobile Communication; see Informatik Spektrum 14 (June 1999) No.3, Berlin, A. Mann, "Der GSM-Standard--Grundlage fur digitale europaische Mobilfunknetze", pages 137 through 152), including the derivative DCS1800 and the American version ADC and Japanese version JDC, as well as in the cordless telecommunications technology according to the DECT standard (Digital European Cordless Telecommunication, see Nachrichtentechnik Elektronik 42 (Jan/Feb 1992), No.1, Berlin, U. Pilger, "Struktur des DECT-Standards", pages 23 through 29), including the American version WCPS, the CT2 and CT3 standard (Cordless Telecommunication).
When building a receiver--for example, for the aforementioned systems--one generally distinguishes between a homodyne receiver (direct receiver) or heterodyne receivers (double detection receivers) with single or double frequency conversion. Compared to heterodyne receivers, the homodyne receiver has the advantage that the homodyne receiver can be more highly integrated. Compared to the homodyne receivers, the heterodyne receiver has the advantages that the selectivity can be easily defined by a band-pass filter at the intermediate frequency and the frequency of the variable oscillator, and that the demodulation occurs at a relatively low frequency. The homodyne receiver, moreover, is not especially well-suited for TDMA systems because the majority of the system amplification is undertaken in the baseband amplifier. These amplifiers, however, react to very low frequency signals and are therefore very sensitive to transient responses that arise due to switching between a transmission mode and a reception mode in the TDMA systems (see ntz, Vol. 46 (1993) No. 10, pages 754 through 757).
FIG. 1 shows a homodyne receiver (direct conversion receiver) disclosed by Great Britain Reference GB-2,286,950 A1 that contains a one-stage synthesizer SYN typical for homodyne receivers with a preceding, low-noise amplifier VS and band-pass filter BPF and with a following limiting means LE and decoder means DE. Two further components (for example, an A-component and a B-component) can be generated with the limiting means LE for an "In Phase" component (I-component) and a quadrature component (Q-component) of the signal to be demodulated, being formed by addition or, respectively, subtraction of the I-component and Q-component. As a result thereof, the angle resolution is enhanced in the complex I/Q level. For the demodulation in the decoder means, further, the components (signals) are limited hard (limited), as a result whereof the statusses "1" or "-1" arise for the I, Q, A and B components.
An angle-keyed signal (for example, the GFSK signal) can have an arbitrary angle in the complex level. The current frequency of the carrier is modified by +.DELTA.f or, respectively, by -.DELTA.f in GFSK modulation for the transmission of digital information. The modification by +.DELTA.f thereby corresponds, for example, to a logical "1", whereas the modification by -.DELTA.f, logically, corresponds to a logical "0". In the complex level, the frequency shift/modification .+-..DELTA.f corresponds to a rotation of the pointer by .DELTA..phi. in clockwise direction (for example, given a logical "1") or, respectively, in counter clockwise direction (for example, given a logical "0"). The amount of the angle change (frequency change) is thereby dependent on the modulation index employed. At least one further coordinate system is generated in order to also be able to identify slight angle changes of the pointer in the I/Q level. For example, this additional coordinate system is formed by the A-component and B-component.
FIG. 2 shows the complex level with the unit circle and two coordinate systems, the I/Q coordinate system and the A/B coordinate system, that are shifted by 45.degree. relative to one another. As a result thereof, the unit circle is divided into eight sectors of equal size. Four quadrants I, II, III, IV in which the pointer can be located can be recognized with each of the two coordinate systems. Two sectors for the possible position of the pointer thus derive in each coordinate system. The actual position of the pointer derives from the meet of two sectors. This is demonstrated with reference to the following example:
A signal to be demodulated or, respectively, decoded exhibits the following status values for the I, Q, A and B components: I=1; Q=1; A=1; B=-1.
According to FIG. 1, the sectors 1 and 2 are possible in this case for the I/Q coordinate system.
According to FIG. 1, the sectors 1 and 8 are possible in this case for the A/B coordinate system. The common meet is the sector 1.
Analogous thereto, the allocations "sector--I,Q,A,B status values" can be identified for the other sectors, these being shown in the following table.
I Q A B Sector No. 1 1 1 -1 1 1 1 1 1 2 1 -1 1 1 3 1 -1 -1 1 4 -1 -1 -1 1 5 -1 -1 -1 -1 6 -1 1 -1 -1 7 -1 1 1 -1 8
All other value combinations are inadmissible.
As already mentioned, the information to be decoded (useful information) is contained in the rotational sense of the pointer. This rotational sense derives from the current position (current sector) and the previous position (previous sector). The current sector must therefore be compared to the previous sector for the demodulation. The rotational sense derives therefrom, as, thus, does the decision as to whether a logical "0" or a logical "1" was sent. The demodulation is thus reduced to the comparison to a table for determining the current sector and a comparison of this sector to the preceding sector.
The reconstruction of the data is possible when the modulation index is big enough in order to see to it that a sector is always transgressed or, stated differently, the plurality of sectors (and, thus, the angle resolution) is to be selected such that the minimum angle change (dependent on the modulation index) always produces a change in sector.
The demodulation of an angle modulated/keyed carrier signal with the receiver disclosed by Great Britain Reference GB-2,286,950 A1 and the above-described demodulation principle, which can likewise be derived from this publication, is only possible for a limited amplitude spectrum of the carrier signal. The reason for this is that the mixers 28, 30 in FIG. 3 of Great Britain GB-2,286,950 A1 limit the signal given certain signal amplitudes, and an evaluation of the amplitude information contained in the I-component and Q-component is therefore no longer possible for generating the A-component and B-component.
European Reference EP-0 637 130 A2 discloses a receiver wherein, due to an automatic gain control (AGC), the carrier signals arriving at a mixer stage are controlled such with respect to their amplitude that a limiting effect does not occur in the mixer stage.
Quasi-homodyne receivers that respectively comprise a two-stage synthesizer have been presented at the ILP Conference, Mar. 9, 1995, University of California, Berkeley, T. Weigandt, S. Mehta, P. R. Gray, "integrated VCO/Synthesizer for DECT/Multi-Standard RF Modems", and at the InfoPad Retreat Conference, Jan. 9-1995, University of California, Berkeley, T. Weigandt, S. Mehta, P. R. Gray, "Frequency Synthesis for a Monolithic CMOS RF Transceiver", whereby a first synthesizer stage is operated with a constant frequency and a second synthesizer stage following the first synthesizer stage is operated with a variable frequency.